Over the past several decades, the number of portable electronic devices has increased dramatically. Concomitantly, there has been a need to provide lighter, rechargeable power sources with increased electrical storage capacity. Although there have been many technological improvements in the fields of photovoltaic cells, fuel cells and supercapacitors, electrochemical storage in the form of a single or multiple electrochemical cells (“batteries”) is still the preferred means to provide power to most portable electronic devices. Electrochemical cells provide an excellent combination of energy capacity, power density and economy. Of the many rechargeable electrochemical cells available in the market today, lithium batteries offer the best performance in terms of specific energy and specific power. At present, the most common rechargeable lithium batteries in use in portable electronic devices are lithium-ion batteries. Lithium-ion batteries combine intercalation electrodes with a non-aqueous liquid electrolyte. Most lithium-ion batteries available today use carbon anodes and cathodes fabricated from metal oxides, phosphates, sulfides or oxysulfides. Another common form of lithium battery in use today is the lithium polymer battery. Instead of a non-aqueous liquid electrolyte, the lithium polymer battery uses a polymeric electrolyte, as its name implies. Some lithium polymer batteries use lithium metal anodes, but the majority use similar electrode materials to lithium-ion batteries.
Lithium-ion and lithium polymer batteries provide premium battery performance at a premium price. However, for many applications, the number of charge-discharge cycles that they can undergo is insufficient. While the cycle life of a capacitor is often measured in millions of cycles, typical cycle lives for lithium-ion and lithium polymer batteries are measured in hundreds of cycles. For applications where large, expensive batteries are required, for example, in all-electric or hybrid-electric vehicles and to back up solar or wind power installations, the cost/lifetime ratio for lithium-ion and lithium polymer batteries is too high. Though some reduction in cost can be achieved by using less expensive materials (such as substituting lithium iron phosphate for the more expensive lithium cobalt oxide cathode material), a way to increase cycle life must be found if lithium batteries are to play a significant role in these large and growing markets.
Another concern often expressed about lithium-ion batteries is their less-than-perfect safety record: there have been a number of highly publicised product recalls associated with “defective” lithium-ion batteries and there are sporadic reports of exploding lithium-ion batteries. These concerns must be addressed and overcome before lithium-ion batteries can be considered for safety-critical applications, e.g., the automotive industry.
Although lithium-ion and lithium polymer batteries have limited charge-discharge cycle lives, another kind of rechargeable lithium battery, the so-called thin-film lithium battery, has a typical cycle life that is measured in thousands of cycles. These thin-film lithium cells comprising thin-film electrodes, electrolytes and current collectors deposited by physical vapor deposition (PVD) onto a substrate have undergone over 20,000 charge/discharge cycles without significant loss of capacity. See W. C. West, J. F. Whitacre and J. R. Lim, “Chemical stability enhancement of lithium conducting solid electrolyte plates using sputtered LiPON thin films”, J. Power Sources, 126, 134-138 (2004). Though thin-film lithium batteries date back to 1969 (see C. C. Liang, J. Epstein and G. H. Boyle, “A High-Voltage, Solid-State Battery System”, J. Electrochem. Soc., 116, 1452 (1969)), this type of battery received much attention when John Bates and his group at Oak Ridge National Laboratory discovered the ‘LiPON’ thin-film electrolyte material in 1994. In contrast to many of the oxide and sulfide based thin-film electrolytes used previously, UPON was stable to metallic lithium and also to cathode materials that exhibited potentials of up to ˜5V versus lithium anodes. It rapidly became the standard solid electrolyte material for thin-film lithium batteries. However, to date, thin-film lithium batteries have yet to find widespread acceptance as energy storage devices. This is primarily due to their very limited energy storage capacity per unit area and the very high cost to make them.
It is difficult to deposit thin-films greater than about 10 microns thick by PVD. The stresses that build up in the thin-films during deposition can cause the films to delaminate or spall from the substrate. For a 10 micron thick cathode fabricated from LiCoO2, a favorite high-performance thin-film cathode material, the theoretical cell capacity is limited to ˜0.8 mA-h cm−2, based on a film density of 5 g cm−3 and the ability to cycle 60% of the Li in LiCoO2. In practice, this probably represents an upper limit for a single thin-film electrochemical cell based on LiCoO2 as 10 micron thick films deposited by PVD are usually significantly less than 100% dense. In U.S. Pat. No. 6,168,884, Neudecker et al. cite a capacity for a thin-film lithium battery with a 1 micron thick LiCoO2 cathode of 69 μA-h cm−2 which correlates well with this estimate. See B. J. Neudecker, N. J. Dudney and J. B. Bates, “Battery with an In-Situ Activation Plated Lithium Anode”, U.S. Pat. No. 6,168,884. Typically, the cost to sputter deposit metal films is ≧0.25 U.S. cents per micron per cm2. To deposit the cathode film alone for a thin-film lithium battery based on LiCoO2 would cost ˜$8,000 per kW-h, which is many orders of magnitude above the targeted figure of $150 per kW-h for automotive and other large energy storage applications. As a result, thin-film lithium batteries must be fabricated using much less expensive thin-film deposition techniques or they will be limited to high-end niche applications with very modest capacity requirements.